Time synchronization protocols, such as Network Time Protocol (NTP) and Simple Network Time Protocol (SNTP), have been extensively studied for traditional Internet and distributed systems. These protocols require multiple handshaking information exchanges in order to minimize estimation of the propagation delay error.
However, in WLANs, propagation delay is not the main source of synchronization errors, and extensive handshaking messages may increase bandwidth consumption. Further, said time synchronization protocols require intensive computation, and consequently, they are inappropriate for deployment in WLANs.
The IEEE 802.11 protocol specification provides a simple clock synchronization solution, wherein an access point (AP) can read the system clock upon generating a beacon frame, and then place timestamp information into the same beacon frame. When stations (STAs) receive a beacon, they can set their system clocks to the value of the timestamp in the beacon to synchronize with the AP. However, this synchronization method is not accurate because it ignores the processing delay of the beacon at the Media Access Control (MAC) and the physical (PHY) layers of both the AP and the STAs, and further ignores the propagation delay through the communication channel between the AP and the STAs.
According to the IEEE 802.11e protocol specification, the MAC layer provides primitives and an interface for a higher layer to perform more accurate time synchronization. This is accomplished by indicating the occurrence of the end of the last symbol of a particular data frame to the higher layer, wherein the higher layer records a timestamp and sends the timestamp through the higher layer data packets. However, the delay jitter caused by interaction between the higher layer and the MAC layer, and between the MAC layer and the PHY layer is still not eliminated or minimized, leading to time synchronization inaccuracy.
There is, therefore, a need for a method and system for accurate clock synchronization for devices communicating in WLANs.